Progress in modern technologies is based on creating new materials and developing optical elements with desired properties based thereupon. In particular, a necessary element in the design of modem displays is an optically anisotropic film possessing the optimum combination of characteristics for a given application.
A number of polymeric materials may be employed in the manufacture of optically anisotropic films. The anisotropic optical properties of these films result from uniaxial extension and modification with organic or inorganic (iodine) compounds. Poly-vinyl alcohol, (PVA) is commonly used as the base polymer as described in Liquid Crystals: Applications and Use, B. Bahadur (ed.), World Scientific, Singapore (1990), Vol. 1, p. 101–103. However, the low thermal stability of PVA-based films limits their applicability. New methods for the synthesis of optically anisotropic films possessing improved characteristics are needed for these reasons.
Organic dichroic dyes may be used for the synthesis of optically anisotropic films that exhibit excellent optical and workability characteristics. Films based on such compounds are obtained through application of a liquid-crystalline aqueous dye solution onto a substrate surface, followed by evaporation of the solvent, such as for instance water. Anisotropic properties may be imparted to the films either through preliminary mechanical orientation of the substrate surface, such as is described in U.S. Pat. No. 2,553,961, or by means of an external orienting action, such as for example mechanical, electromagnetic, or the like, exerted on the film material while it is in a liquid crystal state. This approach is explained in greater detail in PCT patent publication WO 94/28073.
Although the liquid-crystalline properties of dye solutions have been known for some time, extensive investigations of these systems have begun only recently. The new research efforts have been stimulated by the capability of some of these dyes of forming “chromonic” liquid crystal systems. A distinctive feature of chromonic systems is that dye molecules are packed into supramolecular complexes having the form of columns, which are the structural elements of a mesophase. The highly ordered structure of dye molecules in these columns allows use of these mesophases for forming strongly dichroic oriented films.
Molecular structures, phase diagrams, and the mechanisms of molecular aggregation in various chromonic systems, including organic dyes, have been previously reviewed (i.e. Lydon, J. Chromonics, in: Handbook of Liquid Crystals (Wiley-VCH, Weinheim, 1998), Vol. 2B, pp. 981–1007). A special feature of dye molecules that form chromonic mesophases is the presence of peripheral groups that render these dyes water-soluble. The main structural unit of all chromonic mesophases is a column of stacked molecules. The chromonic mesophases of organic dyes are soluble, possess a special structure, and are characterized by specific phase diagrams and optical properties.
By using dichroic dyes capable of forming lyotropic liquid crystal (LLC) systems, it is possible to obtain films possessing a high degree of optical anisotropy. Such films exhibit the properties of E-type polarizers, related to peculiarities of the optical absorption of the chromonic supramolecular complexes. These films behave as retarders (phase-shifting devices) in the spectral regions where the absorption is absent. The phase-shifting properties of these anisotropic films are related to their birefringence which is a difference in refractive indices measured in the direction of application of the liquid-crystalline solution onto a substrate and in the perpendicular direction. These properties of LLC systems account for the growing interest in these materials. New methods are under development to obtain films based on such organic dyes. Recent progress has included both optimization of the film application conditions and identification of new LLC compositions. In particular, new LLC compounds for the synthesis of optically anisotropic films may be obtained by introducing modifiers, stabilizers, surfactants, and other additives into the known compositions so as to improve characteristics of the films. More detailed discussions of these processes are provided in Russian Patent RU 2047643 and published PCT patent application WO 99/31535.
In recent years, the demand for optically anisotropic films characterized by selectivity with respect to various wavelength has increased. Because of the need for films with maximum absorption that may be varied throughout a broad spectral range, from the infrared (IR) to the ultraviolet (UV), there is a strong desire for a broad assortment of compounds capable of forming LLC phases and films possessing required properties. In this context, increased attention has been directed to birefringent film (retarders) materials that are applicable for liquid crystal displays and telecommunication lines. Additional background information on these topics is available in Yeh, P. Optical Waves in Layered Media, (John Wiley & Sons, New York, 1998); and in Yeh, P. and Gu, C. Optics in Liquid Crystal Displays (John Wiley & Sons, New York, 1999). Ultrathin birefringent films can be obtained by forming optically anisotropic layers of liquid crystal systems based on organic dyes. This process has been described in P. Lazarev and M. Paukshto, “Thin Crystal Film Retarders” (2000), Proceeding of the 7th International Display Workshop on Materials and Components (Kobe, Japan, November 29–December 1), pp. 1159–1160, wherein thin optically anisotropic crystalline films based on disulfonic acid esters of Vat Red 14 dye were obtained. The films included a mixture of cis- and trans-isomers of naphthalenecarboxylic acid dibenzimidazoles with the following structural formulas:
Using this technology, it is possible to control the crystallographic axis direction in the film in the course of application and crystallization on a substrate. The films, obtained on glass plates with dimensions 5 by 7.5 cm, possessed homogeneous composition and high crystalline order and were characterized by a dichroic ratio Kd equal to 28. Such films can be used as polarizers and retarders.
As FIG. 1 shows, the oriented films based on Vat Red 14 dye exhibit a high degree of optical anisotropy, as characterized by a large difference of the refractive indices for the ordinary and extraordinary rays: no−ne=0.6–0.8 for l=550–700 nm. Use of these films as retarders is restricted to a green spectral range, where the dye does not absorb light.
Thin birefringent films that are transparent in the visible range may also be based on disodium chromoglycate (DSCG), a compound with the structural formula:
The degree of optical anisotropy of an oriented film of this compound is not as large. The difference of the refractive indices amounts to approximately 0.1–0.13. However, since the thickness of DSCG layers can be varied within broad limits, it is possible to achieve a desired phase shift even with this relatively low. The main disadvantage of DSCG films is their temporal instability, which is manifested by gradual recrystallization and a decrease in the degree of optical anisotropy.
Various compositions based on water-soluble organic dyes can be also used for obtaining optically anisotropic films according to the above technology as illustrated in PCT publications WO 94/28073 and WO 99/31535, However, a common disadvantage of these materials is high absorption in the visible spectral range. This property significantly restricts the use of these dyes for the synthesis of transparent birefringent films.